What is an Okazaki fragment and on which strand is it synthesized?

• A. Short fragments of DNA synthesized on the leading strand.
• B. Short fragments of DNA on the leading strand.
• C. Short fragments of DNA synthesized on the lagging strand.
• D. RNA transcripts.

Answer: (C)

Okazaki fragments are short segments of DNA that are synthesized on the lagging strand during DNA replication. DNA polymerase can only add nucleotides in the 5' to 3' direction, and because the strands are antiparallel, the strand that runs 5' to 3' away from the replication fork cannot be copied continuously. Instead, it is built discontinuously as many short pieces—the Okazaki fragments. Each fragment begins with an RNA primer laid down by primase, then DNA polymerase extends it, and later the fragments are joined together by DNA ligase to form a continuous strand. This is in contrast to the leading strand, which is synthesized continuously toward the fork. The fragments themselves are DNA, not RNA transcripts.